The present invention relates to polarization-independent optical devices and to a method for polarization independent processing of a signal. Different optical devices, as for example optical amplifiers and optical filters or combined optical, amplifying filters are used for example in optical communication systems, e.g. in integrated components for optical-signal-processing. An important problem with these devices resides in that they generally are polarization-dependent. This means particularly that the device has different influence on the parts of an input signal which are TE-polarized and TM-polarized respectively. This means e.g. for an optical amplifier or a laser amplifier that the gain will be different for TE-polarized and TM-polarized waves respectively; the difference in gain between the differently polarized waves can under certain circumstances be of several decibels which gives rise to significant problems. The problem resides thus in that for incident signals with different polarization states, the modes experience an optical device, for example an amplifier or a reflection filter or a combination of both, in different ways. A consequence thereof is, besides impaired amplification qualities or reflection properties in general, problems in the form of impaired properties upon use together with conventional monomode-fibres or other components which do not preserve the polarization state of the signal. Generally it can be said that the polarization-dependence of optical bulk laser amplifiers is explained by the active wave guide having an asymmetric cross-sectional geometry or the active layers being asymmetric. This asymmetry gives rise to differences in optical confinement factors, effective refractive indices and facet-reflectivities for the transversal electrical and magnetical modes respectively of the wave-guide, in this context those modes of the wave guide are meant for which the predominating part of the electrical and magnetical field strength vector respectively is parallel with the horizontal plane and perpendicular to the direction of propagation. These differences are particularly pronounced when common laser-diode-structures are used as amplifiers since those often have an active layer, the thickness of which is considerably smaller than the width of the same. The polarization-sensitivity makes the optical amplifiers incompatible with systems using conventional monomode fibres which do not preserve the state of polarization. So called distributed Bragg-filters are polarization-dependent mainly due to the difference in propagation constants, i.e. effective indices for the TE-, TM-modes.
A large number of solutions to the above mentioned problem have been suggested. According to one solution, as given in M. Sumida: "Polarisation insensitive configuration of semiconductor laser amplifier", Electron. Lett., vol. 26, p. 1913-1914, 1990 a combined so called splitter/combiner is used which splits up incident light in s-, and p-polarized beams, each beam going through a so called Faraday-rotator with a rotation angle of 45.degree. whereafter they go through a polarisation maintaining fibre whereupon they are injected into a laser-amplifier. In this they are amplified with the same TE-mode gain, propagating through the rotation maintaining fibres and finally they go through the Faraday-rotators. Finally, the s-, and p-polarised beams respectively are combined in the combiner. In this device the amplifier itself is polarisation-dependent but the device taken as a whole, behaves, seen from the outside as a polarisation-independent device. Another suggestion of a solution to the problem is given in "Polarisation-independent configuration optical amplifier", Electron. Lett., vol. 24, p. 1075-1076, 1988 by N. A. Olsson. Therein is described how polarisation-independent gain is achieved through making the input signal pass a semiconductor-laser amplifier twice, the signal after the first passage going through a so called Faraday-rotator with a rotation angle of 45.degree., is reflected and whereupon it again passes the Faraday-rotator whereafter it for the second time goes through the laser amplifier with a polarisation which has been rotated 90.degree.. Even in this case the amplifier itself is polarisation-dependent whereas the polarisation is controlled and rotated respectively by separate units. According to another known embodiment as disclosed by G. Grosskopf, R. Ludwig, R. G. Waarts, H. G. Weber in "Optical amplifier configurations with low polarisation sensitivity", in Electron. Lett., vol. 23, p. 1387- 1388, 1987, instead two separate amplifiers are used in combination. Thereby is described how the amplifiers either can be arranged in series or in parallel. In the case of coupling in series an optical wave with TE-polarisation in amplifier 1 has TM-polarisation in amplifier 2 and vice versa, and if both amplifiers exhibit equal gain properties, a polarisation-independent system is achieved. In the case of amplifiers arranged in parallel, the input signal first has to go through a polarisation splitter. Even in those cases the amplifiers are thus polarisation dependent whereas the system seen from the outside is polarisation-independent. It is also known to use amplifiers connected in series with a polarisation insensitive isolator arranged inbetween which rotates the polarisation 90.degree.. This is described in "Polarisation insensitive optical amplifier consisting of two semiconductor laser amplifiers and a polarisation insensitive isolator in series", IEEE Phot. Technol. Lett., vol. 1, p. 431-433, 1989 by M. Koga, T. Matsumoto. It is furthermore known to make active wave guides, the thickness of which being essentially the same as its width, which may get similar properties for TE-and TM-polarised signals respectively. According to a further known embodiment, so called strained multiple quantum wells (MQWs) are used in an active wave guide. (Disclosed in "Polarization insensitive travelling wave type amplifier using strained multiple quantum well structure", IEEE Phot. Technol. Lett., vol. 2, p. 556-558, 1990 by K. Magari et. al.)
However, none of these devices solves in a satisfactory way the above mentioned problems. The first-cited solutions require external components. This gives rise to a complex system which is therefore expensive and under certain circumstances also sensitive to disturbances and leads to difficulties upon integration for example in communication systems. In the case of fabrication of wave-guides where the thickness and the width are comparable it is required that a symmetrical cross-section is achieved in order to get the same gain-charactistics for TE- and TM-modes respectively. Finally the amplifier with Strained MQWs works satisfactory only for one gain level. For polarisation independent filters the wave guides have to get the same effective index for the TE- and TM-modes respectively. According to a known embodiment this has been achieved through use of very small index steps. This however leads to a poor flexibility upon forming of wave guides which in turn may lead to problems for example upon monolithic integration with other components. (See for example "Bragg gratings on InGaAsP/InP wave guides as polarisation independent optical filters", J. Lightwave Technol., vol. 7, p. 1641-1645, 1989 by C. Cremer et al).